Extrusion apparatus for the preparation of instant fresh corn dough or masa

ABSTRACT

Instant fresh corn dough or masa may be prepared in short processing times, with low energy consumption and without liquid wastes by extruding a mixture of corn flour, lime and water in an extrusion or kneading chamber until a homogeneous dough is obtained, heating the homogeneous dough to cooking temperature while kneading is continued, expelling the cooked dough through a die from the extrusion chamber to a holding chamber, cooling the dough in said holding chamber and discharging the cooked and cooled dough through a discharge orifice, whereby the total processing time may be reduced to between 1.5 to a maximum of 7 minutes for the preparation of a fully nixtamalized corn dough or masa.

This is a divisional of application Ser. No. 08/326,519, filed Oct. 20,1994.

FIELD OF THE INVENTION

The present invention refers to an apparatus and a continuous method forthe manufacture of fresh corn masa (nixtamalized corn dough) and, moreparticularly, it is related to an extrusion apparatus and a continuousmethod for the manufacture of fresh corn masa for use in the preparationof tortillas, instant masa flours and derivatives thereof.

BACKGROUND OF THE INVENTION

The history of the corn tortilla is lost in the pre-Columbian time withthe ancestors of the native Aztec and Maya peoples. The basic steps inmaking tortillas has continued unaltered since these times. In fact, thetraditional process employed by the first inhabitants of Mesoamerica isstill been used today.

Corn tortillas are one of the most important food products of Mexico andof some Central American countries. It constitutes the major source ofproteins and calories for the lowest social economical groups. At thepresent time, there is a large expansion in the consumption of tortillasin the U.S.A. as well. Tortillas are also employed as the raw materialfor the preparation of innumerable traditional food products such assnacks, tortilla chips, taco shells, tostadas, enchiladas, burritos andnachos, all of which are extremely popular in the southern states of theU.S.A. In Mexico and some Central American countries, tortillas, undervarious types of products, constitute a significant part of the diet ofevery social strata, with the average annual per capita consumption of182 Kg. In rural areas they provide approximately 70% of all caloriesand 50% of all proteins ingested daily.

The traditional method of nixtamalization consists in the alkalinecooking of whole corn in water to which 1 to 2% by weight of lime hasbeen added, by heating the mixture to boiling for 30-40 min. andsteeping the cooked mixture for 12 to 15 hr. The steeped liquor rich insolids (called nejayote) is discharged. The cooked steeped maize (callednixtamal) is washed to remove excess of lime and lost pericarp tissue.The nixtamal is ground with a pestle and stone into dough (called masa)and flattened into thin disks that ape baked for 30-60 sec on each sideon a hot griddle (comal) heated to 180° to 210° C.

In industrial processes corn is cooked using several different cookingand steeping processes. Important process variables are: the type ofcorn, the cooking time and temperature, the lime concentration, theagitation frequency (which is necessary to keep the lime suspended) andthe nixtamal washing procedures. A common procedure for cooking cornuses steam kettles which contain water, lime and corn, into which steamis injected until a temperature near boiling is reached. The cookingwater is circulated and the suspension is stirred to suspend the lime.To be used for the manufacture of tortillas the corn is held at nearboiling for 30 to 40 minutes; for other uses, such as the manufacture oftortilla chips less time is required.

After cooking, cold water is added and the mixture is steeped for 8 to12 hr. Afterwards the nixtamal is washed generally using horizontalrotating barrels or drums with the addition of pressurized water in theform of a spray. For the production of fresh masa the nixtamal is washedagain and then milled. To prepare tortillas the masa is shaped intocircular pieces and baked, generally in a gas oven with conveyor beltsheated to a temperature of between 300° and 320° C.

The traditional commercial process for the production of tortillas anddry masa flours described above as admitted prior art involves excessivetime and large floor space, as well as the production of liquid waste.In addition, as a result of the process important nutrients such asthiamin, niacin, riboflavin, fat and fiber are lost. The liquid wastefrom the cooking process, or nejayote, contains solid particlesconsisting of fragments of the pericarp (fiber), starch, protein andsoluble germ. In addition to the above drawbacks, in some casessubjective determinations and empirical methods used in each step of theprocess may affect the quality of the final product. All these factorshave important economical and commercial implications.

Several research works had been conducted with the objective of makingthe traditional nixtamalization process more efficient. These mainlyconsider relationships between concentration of water, corn and lime,the cooking temperature and time, the steeping time of the cooked cornand its milling, without modifying the basic characteristics of thetraditional nixtamalization process.

Such works have been described, for instance, in U.S. Pat. No.2,584,893, patented on February 1952 to Lloyd, W. R. and MillaresSotres, R., which discloses a method for making tortilla flour thatincludes a stage that comprises the traditional nixtamalization processdescribed above.

U.S. Pat. No. 2,704,257, patented on Mar. 15, 1955 to Diez de Sollano,C. S. F. and Berriozabal, J. M. discloses a method for producing corntortilla flour which includes an improvement over the traditionalnixtamalization process, by directly converting the wet nixtamal into adry flour in one single milling and drying step. The traditional cookingand steeping steps, however, continue to be carried out prior to themilling and drying of the nixtamal corn grains, although the steepingstep is carried out by the use of perforated drums within which the cornkernels are contained, said drums being immersed into a steeping bathcontaining the calcium oxide. The main advantage of this method is thatof omitting the obtention of a corn dough or masa for furthertransformation into a flour. The drawbacks shown by the ancient cookingand steeping steps, however, remain substantially unchanged.

Anderson, E. E. and Brown, J. D. describe in U.S. Pat. No. 3,083,103,patented on Mar. 26, 1963, a method that allegedly reduces the treatmenttime for corn kernels to one tenth as compared with the traditionalnixtamalization process, However, Anderson et al use a supersaturatedcalcium hydroxide solution as the steeping liquor and carry out thesteeping operation under high pressure, which obviously require aspecial type of costly equipment. On the other hand, the supersaturatedalkaline steeping bath renders the control of the alkalinity ratherdifficult and frequently produces an excessively alkaline nixtamal whichwill create an off-flavor in the finished product. Therefore, thismethod has not gained popularity in the market.

Eytinge, B. D., U.S. Pat. No. 3,194,654 patented on Jul. 13, 1965,discloses a method for the production of nixtamal which, withoutdeparting from the traditional nixtamalization process, renders saidprocess continuous by providing a steeping receptacle at the top ofwhich the corn kernels are continuously fed and at the bottom of whichan alkaline steeping solution is fed in counter-current with saidkernels and at a suitable cooking temperature to accomplish thenixtamalization of the corn in a time of approximately 5 hours. Althoughthis method represents an improvement over the traditionalnixtamalization process, it requires the use of very costly equipmentthat is difficultly controllable.

U.S. Pat. No. 3,404,986, patented on Oct. 8, 1968 to Wimmer, E. L.,Grove, E. and Susek, J. L., discloses a method for producing corn flourssuitable for fried snacks, but which would be absolutely unsuitable forthe manufacture of tortillas and the like. Wimmer et al use a mechanicalmethod for separating the germ, endosperm and hull of the kernel andthen prepare mixtures of whole corn meal and endosperm enriched meal andsubject it to the action of hot steel rollers to cause starchgelatinization and drying of the meal. Although a masa flour is obtainedby this process which can be used for fried snacks, it is not useful forthe manufacture of tortillas in view of the absence of an alkalinestooping step which will soften the masa flour as is necessary formaking tortilla flour.

Celorio Mendoza, F., U.S. Pat. No. 3,859,452, patented Jan. 7, 1975,describes a method for nixtamalizing whole corn flours by passing amixture of said flour and lime upwardly through a vertical treating zoneby means of an ascending air stream, heating the mixture to releasesteam from the flour, thus wetting the lime in order to react with theflour and nixtamalize the same, and passing the humid air entrainedmixture to a cooling zone to cause reabsorption of the water. Thisvariation of the traditional nixtamalization process, as it may beeasily seen, lacks means for controlling the uniformity of thenixtamalization of the treated flour and therefore is incapable orproducing a nixtamalized flour with sufficient uniformity for use in themanufacture of tortillas.

Hart, E. R., U.S. Pat. No. 4,329,371, patented on May 11, 1982,describes a method of obtaining corn masa without the use of thealkaline nixtamalization, by providing a step of dehulling the corngrains by means of an extremely complex hydraulic dehulling apparatus,separating the hull from the endosperm and germ portion of the grain,using the hull for other applications, and cooking the endosperm andgerm portion with steam. Although this method is capable of obtaining amasa flour perfectly suitable for the manufacture of tortillas, it showsthe drawback that the hull must be discarded or used for other purposesof lower economical value.

Tonella, M. L., Sanchez, M. and Salazar, M. G., in Journal of FoodScience, Volume 48, 1983, describe on page 1637 a method for obtainingnixtamalized corn flour which uses a slight modification of thetraditional nixtamalization process by cooking the suspension of cornand lime at a slightly higher temperature and steeping the suspensionfor about 24 hours, to then rinse with water and dry the kernels in atunnel drier and mill the dried kernels in a Ciclotex mill. This methoddoes not depart from the traditional nixtamalization process andtherefore does not represent an advance in the art.

U.S. Pat. No. 4,513,018, patented on Apr. 23, 1985 to Rubio, M. J.,discloses a continuous method for producing corn flour suitable formaking tortillas, taco shells, tortilla chips and the like, which uses amodified nixtamalization process comprising the precooking of corngrains in the presence of lime within a receptacle provided with a screwconveyor. The precooked kernels are then treated with a spray of hotwater and are passed to a cooling zone where they remain for a timesufficient to reabsorb a sufficient amount of water. The thus steepedgrains are milled and the flour is classified and re-milled until thedesired particle size is obtained. This method, although representing animprovement over the traditional nixtamalization process, still must usethe same by cooking the grain in an alkaline bath for a considerableperiod of time.

Vaqueiro, H. C. and Reyes, P. disclose, in U.S. Pat. No. 4,594,260,patented on Jun. 10, 1986, a method of obtaining corn flour including aparticular nixtamalization stage which essentially comprises removingthe hull from the germ and endosperm of the corn kernel by humidifying,crushing and sieving the integral corn kernels until a fractioncontaining hull and a fraction containing endosperm and germ areobtained, and then nixtamalizing by the traditional method only the hullfraction and re-mixing the same with the non nixtamalized endosperm andgerm fraction. This method, although more efficient than the traditionalmethods in view of the fact that only a small fraction of the cornkernel has to be nixtamalized, requires a rather elaborate equipment andtakes a relatively long time to effect full nixtamalization of the hullfraction,

As it may be seen from the above, the prior art methods for theproduction of corn dough or masa involve excessive time and large floorspace, as well as the production of large amounts of liquid waste. Inaddition, important nutrients such as thiamin, niacin, riboflavin, fatand fiber are lost.

With the aim of solving the above described problems shown by thetraditional nixtamalization process and the modifications thereof, someworkers have developed certain manners of cooking by extrusion cerealgrains. Thus, Matsumoto, Y. and Taguchi, G. describe in U.S. Pat. No.4,748,037, patented on May 31, 1988, a cooker-extruder which utilizesthe grains as they are, without a previous crushing step, and treatssaid grains, under controlled humidity conditions, by means of atwin-screw extruder under a relatively high temperature and expellingthe crushed cooked grain through a die. Although corn kernels arementioned as one of the cereals which may be treated in thiscooker-extruder, the description is not enabling towards said grains,and it may be assumed that the corn grains must be used after adehulling operation is carried out, because the conditions of cookingand extrusion would not be able in themselves to nixtamalize the hullsof corn grains.

In U.S. Pat. No. 4,756,921, patented on Jul. 12, 1988 to Calandro, T.,Straks, R. and Verrico, M., a similar twin-screw cooker extruder isdescribed, in which different types of cereal brans are used, includingcorn (maize) bran. This cooker-extruder works under high shear forceswhich unduly heat the material under treatment, which obviously bringsabout difficulties in the control of the process in order to avoidscorching, and is generally intended for treating mixtures of bran andsyrup by means of a high shear blending zone, a cooking zone and acooling zone, from which the paste is discharged through a die havingperforations predetermined size and shape in order to directly formcereal nuggets.

U.S. Pat. No. 4,769,251, patented on Sep. 6, 1988 to Wenget, M. L. andHuber, G. R. describes a cooker-extruder specially designed forobtaining cooked rice products and comprising a preconditioning zonewhere the rice grains are precooked, a cooking zone where the mass ofgrains are heated, a vent or vacuum zone to apply a vacuum to cool therice, and a forming zone containing a die of predetermined size andshape to obtain an extruded rice product. This cooker-extruder, however,is not suitable for the treatment of corn in view of the rather specialcharacteristics thereof.

Finally, U.S. Pat. No. 4,985,269, patented on Jan. 15, 1991 to Irvin, S.A., Foder, R. A. and Merritt, C. G., describes a twin-screw high shearcooker extruder to which corn kernels and lime-water are fed. Theextruder contains a first section where mixing and blending is effectedunder low shear and low compression, a second section which is one ofintense mixing, shearing and kneading and where cooking is carried outby the provision of heat mainly originated from the high shear forces towhich the dough is subjected, supplemented by heat from an externalsource, and a third low shear section where the dough is cooled prior todischarge through the die of the extruder in order to avoid expansion ofthe dough after leaving the die. This extruder is highly suitable forthe production of a rather compact, non expanded dough useable for thepreparation of tortilla chips, but which could hardly be used for theproduction of puffed tortillas.

Referring in general terms to the above discussed prior artcooker-extruders, it is to be noted that the screw is probably the mostimportant part of the extruder as it governs not only the degree ofcooking and gelatinization but also the quality of the final product.The most drastic changes in the cooking-extrusion process are due tochanges in the geometry of the screw. The performance, efficiency andcapabilities of an extruder depend to a large extent on the design ofthe screw and the screw sleeves which govern the flow mechanism of thematerial in the extruder. Generally speaking, in the prior art extrudersmentioned above, the screw is divided into several zones with differingpressure and with a compression ratio higher than 1, and where the shearand pressure are increased in order to accelerate the cooking of theproduct.

Although the above described prior art cooker-extruders are capable ofproducing either-instant corn flour or instant corn dough or masa invery short times and with a quality suitable for the obtention oftortilla chips and the like, they are not suitable for obtaining a softdough having the special characteristics necessary to produce tabletortillas or even taco shells, whereby an extruder capable of producingsuch a nixtamalized corn dough or masa was still unavailable in themarket.

OBJECTS OF THE INVENTION

Having in mind the defect of the prior art methods and extruders for thepreparation of corn dough or flour, it is an object of the presentinvention to provide a continuous method for the preparation of freshcorn dough or masa, which will be capable of producing a high qualitydough with the use of short processing times.

It is another object of the present invention to provide a continuousmethod of the above described character, which will require the use ofsmall-amounts of processing water and will have low energy requirementsfor a high yield of the product.

One other object of the present invention is to provide an extrusionapparatus for the continuous production of fresh corn dough or masa,which will be of a relatively simple construction and yet of a highyield of the product.

A more specific object of the present invention is to provide anextruder of the above mentioned character, which will have a low energyconsumption per unit of product.

Another object of the present invention is to provide an extruder of theabove mentioned character, which will facilitate the incorporation ofnutrients and additives in the dough.

The foregoing objects, as well as others ancillary thereto arepreferably accomplished as follows:

According to a preferred embodiment of the present invention, acontinuous method for the obtention of fresh corn dough or masacomprises the steps of:

a) milling corn kernels in order to obtain a corn flour having aparticle size suitable for the manufacture of tortillas;

b) admixing the thus obtained flour with lime and water in amountssufficient to provide from about 25 to about 60% by weight of moistureand a pH of from about 6 to about 7.5;

c) subjecting the mixture to kneading by extrusion under a relativelylow compression and shear rate for a period of time of from about 0.5 toabout 5 minutes;

d) heating the kneaded dough to a temperature of from about 60° to about95° C. for a period of time of from about 1 to about 2 minutes;

e) cooling the heated and cooked dough until a temperature of from about40° to about 70° C. is reached; and

f) expelling the cooled dough through a die in order to obtain ahomogenized fresh corn dough or masa.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features that are considered characteristic of the presentinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its organization and its method ofoperation, together with additional objects and advantages thereof, willbest be understood from the following description of a specificembodiment when read in connection with the accompanying drawing, inwhich:

FIG. 1 is a schematic cross sectional elevational view of acooker-extruder built in accordance with the principles of the presentinvention.

DETAILED DESCRIPTION

Having now more particular reference to the drawing, there is shown acooker extruder built in accordance with the present invention, whichessentially comprises a feed hopper 1 preferably provided with a stirrer(not shown) to receive and admix corn flour obtained by milling cornkernels, lime and water in suitable proportions; a cylindrical extrusionor kneading chamber 2 provided with a screw conveyor 9 which may be ofthe single or multiple screw type, said extrusion chamber 2 beingprovided with a constant depth channel or barrel 3 to contain the screwconveyor 9, and the latter being of a design and having a rotationalspeed suitable for providing only a low shearing of the material andalso a low compression ratio, which may vary from 1:1 to 1.5; a heatingjacket 10 surrounding the discharge portion of the extruder 2, in orderto form a cooking section 6; a die or matrix 12 placed at the dischargeend of the screw conveyor 9 and provided with a suitable orifice 13 fordischarging the dough at a suitable rate and at the same time forproviding a counterpressure to the screw 9 in order to maintain thedesired degree of compression throughout the extruder; a holding chamber4 provided with a cooling jacket 5 provided with heat dissipation finssuch as indicated at 11; and a die 7 having a discharge nozzle 8provided at the discharge end of the holding or cooling chamber 4.

Although it is preferred to provide a screw 9 of the single screw typehaving a constant pitch and a cylindrical shape for the apparatus of thepresent invention, it is perfectly possible to use, without detriment ofthe quality of the product obtained, a screw of the multiple screw typeand having a tapered shape, provided that a changing pitch is providedin order to maintain a constant low shear and a constant compressionratio throughout the length of the screw.

In order to produce fresh corn dough or masa of a quality suitable formaking tortillas, whole or dehulled corn grains are milled to obtain aflour having a particle size distribution appropriate for tortillas andderivatives which are based on fresh masa. The preferred particle sizedistribution for the above purpose is of between 0.1 to 0.5 mm,depending on the subsequent use desired for the corn dough obtained bythe method of the present invention. The raw corn flour obtained is thenhomogeneously mixed in the feed hopper 1 with sufficient amounts of limeand water, for instance, by the introduction into said hopper 1 of asuitable stirrer of kneader (not shown), said amounts being sufficientto obtain a mixture containing from about 25 to about 60%, andpreferably from about 50 to about 60% by weight to moisture and a pH offrom about 6 to about 7.5. In order to obtain the above pH, it isgenerally necessary to add to the corn flour a proportion of from about0.1 to about 2.5% of lime. It is to be noted that if a dehulled corngrain is used, lower amounts of lime will be required as compared tothose that must be used with whole corn grain flour, since thenixtamalization of the endosperm and germ with only minor proportions ofhull, requires less energetic conditions. On the other hand, if thefresh corn dough to be produced is intended for the preparation of cornflour, then lower amounts of water are convenient in order to avoid theuse of large amounts of energy to evaporate the water.

The mixture is then admitted in the channel 3 of the extruder 2, whereit is subjected to the action of the screw 9, which is preferably astraight screw of the single screw type, although multiple screw typeconveyors may also be used, said screw 9 being rotated at a speed suchthat a low shearing action is exerted on the mixture and the channel 3is provided with a constant depth in order to produce a compressionratio not higher than 1:1.5 and preferably of from about 1:1 to about1:1.5. The rotational speed of the screw 9 is such that the mixture willneed from about 0.5 to about 5 minutes to pass through the length of theextruder 2 prior to entering the heating zone 6. The barrel of theextruder 2 is preferably provided with longitudinal grooves throughoutthe length of the inner wall thereof in order to prevent the dough toslip on said inner wall, with which a uniform shearing action isobtained.

The screw 9 conveys the mass to the heating zone 6, where the dough isheated to a temperature of from about 60° to about 95° C. in order toproduce the required degree of gelatinization to make a fresh masa ofgood characteristics. The heating jacket 10 is operated so as to supplyheat only sufficient to complement the heat produced by the shearing andcompression forces produced by the screw 9, such that the abovementioned temperature be obtained. The residence time of the doughwithin the heating zone 6 is of from about 1 to about 2 minutes. Thisstage is the most critical of the method, since small variations in theamount of heat generated in or supplied to the corn dough may cause,with excess cooking, the formation of a sticky and gummy dough or, withinsufficient cooking, the formation of a crumby and off-taste dough.

The nixtamalized corn dough thus produced is then discharged at the endof the screw 9 through the orifice 13 of the die 12 to the channel ofthe holding zone 4 where, with the aid of the cooling jacket 5, thedough is cooled to a temperature of from about 40° to about 70° C.,preferably of between 45° and 60° C. and most preferably between 50° and55° C.

The cooled nixtamalized fresh corn dough or mass is thereafterdischarged from the cooker-extruder through a suitable die 7 containinga nozzle 8 for further use in the manufacture of tortillas, nixtamalizedcorn flour, tortilla chips, snacks and the like. The tortillas obtainedwith this fresh masa generally have a moisture content of 40-50%.

In accordance with the above description of the cooker-extruder of thepresent invention, as well as its mode of operation, it may be concludedthat the method for the preparation of nixtamalized fresh corn dough ormasa in accordance with this invention essentially comprises thefollowing steps:

a) milling whole or dehulled corn kernels in order to obtain a cornflour having a particle size suitable for the manufacture of tortillas,such as a particle size of from about 0.1 to about 0.5 mm;

b) admixing the thus obtained flour with lime and water in amountssufficient to provide from about 50 to about 60% by weight of moistureand a pH of from about 6 to about 7.5;

c) subjecting the mixture to kneading by extrusion under a relativelylow compression and shear rate for a period of time of from about 0.5 toabout 5 minutes;

d) heating the kneaded dough to a temperature of from about 60° to about95° C. for a period of time of from about 1 to about 2 minutes;

e) cooling the heated and cooked dough until a temperature of from about40° to about 70° C. is reached; and

f) expelling the cooled dough through a die in order to obtain ahomogenized fresh corn dough or masa.

The present invention will be more fully understood by way of thefollowing examples, that are given with a merely illustrative and nonlimitative purpose.

EXAMPLE 1

This example is intended to describe the method followed to obtain freshmasa and tortillas from whole corn kernels. Initially clean raw wholecorn kernels were milled and the whole corn flour obtained was allowedto flow by gravity into a mixing device providing a uniform andhomogeneous admixing action, where water and lime (calcium hydroxide) inamounts sufficient to provide 60% by weight of moisture and 0.5% byweight of calcium hydroxide to the mixture were added. The resultingmixture, having a moisture content of 60% by weight and a pH of 7.5 wasthen fed into a cooker-extruder of the present invention.

The flights of the screw of the extruder conveyed the mixture down thebarrel thereof, pressing it at a compression rate of 1:1.2 through a diewhich permitted the passage of the masa into the holding chamber whichhad at its exit end an orifice through which the fresh masa wasdischarged. The screw speed was of 35 rpm, and the temperature of thecooking zone was 80° C. The moisture content of the fresh masa thusobtained was of 58% by weight. The residence time of the materialswithin the apparatus was of 1.5 minutes. A fresh masa having very goodplasticity and consistency was obtained.

The fresh masa was used to prepare tortillas that had a moisture contentof 53% by weight, a pH of 7.0, a protein content of 10.0 and an ashcontent of 1.3%.

EXAMPLE 2

This example is intended to describe the method followed to obtain freshmasa and tortillas from dehulled corn kernels. Whole corn kernels (withits inherent moisture content) were subjected to an attrition process inorder to partially remove pericarp and germ, leaving approximately 7% ofexternal layers. The dehulled grains were milled and the flour obtainedwas allowed to flow by gravity into a device providing a uniform andhomogeneous admixing action, where water and lime (calcium hydroxide)were added in amounts sufficient to provide 60% by weight of moistureand 0.2% by weight of calcium hydroxide. The resulting mixture,containing 60% of moisture and a pH of 7.0 was delivered into theextruding chamber of the cooker-extruder of the present invention.

The flights on the screw of the extruder conveyed the mixture down thebarrel thereof pressing it under a compression rate of 4:1 through a diewhich permitted the passage of the masa into the holding chamber whichhad at its discharge end an orifice through which the fresh masa wasdischarged for preparation of tortillas. The screw speed was of 35 rpm,and the temperature of the cooking zone was of 80° C. The moisturecontent of the fresh masa was 58%. The residence time of the rawmaterials within the apparatus was 1.5 minutes.

The fresh masa was used to prepare tortillas that had a moisture contentof 53%, a pH of 7.0, a protein content of 9.7% and an ash content of1.2%.

EXAMPLE 3

This example is intended to describe the method followed to obtain freshmasa and instant masa flours to prepare tortillas, from whole cornkernels. Initially clean raw whole corn kernels were milled and thewhole corn flour obtained was allowed to flow by gravity into a mixingdevice providing a uniform and homogeneous admixing action, where waterand lime (calcium hydroxide) in amounts sufficient to provide 30% byweight of moisture and 0.5% by weight of calcium hydroxide to themixture were added. The resulting mixture, having a moisture content of30% by weight and a pH of 7.5 was then fed into a cooker-extruder of thepresent invention.

The flights of the screw of the extruder conveyed the mixture down thebarrel thereof, pressing it at a compression rate of 1:1.2 through a diewhich permitted the passage of the masa into the holding chamber whichhad at its exit end an orifice through which the fresh masa wasdischarged. The screw speed was of 35 rpm, and the temperature of thecooking zone was 80° C. The moisture content of the fresh masa thusobtained was of 28% by weight. The residence time of the materialswithin the apparatus was of 1.5 minutes. A fresh masa having very goodplasticity and consistency was obtained.

The fresh masa was cut into small pieces, milled and dehydrated toobtain an instant whole masa flour, which was then rehydrated to preparetortillas. The instant whole masa flour obtained in this example had amoisture content of 10.5%, a pH of 7.0, a protein content of 10.0% andan ash content of 1.3%.

EXAMPLE 4

This example is intended to describe the method followed to obtain freshmasa and instant masa flours to prepare tortillas, from dehulled cornkernels. Whole corn kernels (with its inherent moisture content) weresubjected to an attrition process in order to partially remove pericarpand germ, leaving approximately 7% of external layers. The dehulledgrains were milled and the flour obtained was allowed to flow by gravityinto a device providing a uniform and homogeneous admixing action, wherewater and lime (calcium hydroxide) were added in amounts sufficient toprovide 30% by weight of moisture and 0.2% by weight of calciumhydroxide. The resulting mixture, containing 30% of moisture and a pH of7.0 was delivered into the extruding chamber of the cooker-extruder ofthe present invention.

The flights on the screw of the extruder conveyed the mixture down thebarrel thereof pressing it under a compression rate of 1:1 through a diewhich permitted the passage of the masa into the holding chamber whichhad at its discharge end an orifice through which the fresh masa wasdischarged for preparation of tortillas. The screw speed was of 35 rpm,and the temperature of the cooking zone was of 80° C. The moisturecontent of the fresh masa was 28%. The residence time of the rawmaterials within the apparatus was 1.5 minutes.

The fresh masa was cut into small pieces, milled and dehydrated toobtain an instant masa flour, which was then rehydrated to preparetortillas. The instant masa flour obtained in this example had amoisture content of 10.5%, a pH of 7.0, a protein content of 9.7% and anash content of 1.2%.

Although certain specific embodiments of the present invention have beendescribed above, it must be understood that many modifications thereofare possible. The present invention, therefore, is not to be restrictedinsofar as is necessitated by the prior art or by the spirit of theappended claims.

What is claimed is:
 1. An extrusion apparatus for the preparation offresh corn dough or masa comprising:a feed hopper for receiving andadmixing raw materials to be processed in suitable proportions to formthe corn dough or masa an extrusion or kneading chamber provided with ascrew conveyor having a discharge end, the screw conveyor being operableat a rotational speed suitable for providing shearing of said rawmaterials at a desired compression ratio; heating means surrounding thedischarge end of the extrusion chamber and forming a cooking section forheating said raw materials to a desired temperature; a die located atthe discharge end of the extrusion chamber and provided with a suitableorifice for discharging the raw materials at a suitable rate while atthe same time providing a counterpressure to the screw conveyor in orderto maintain a constant desired compression ratio throughout theextrusion chamber; a holding chamber connected to the discharge end ofsaid extrusion chamber, the holding chamber having a discharge end andbeing provided with cooling means for cooling the holding chamber; and adischarge die having a discharge nozzle provided at the discharge end ofthe holding chamber.
 2. An extrusion apparatus according to claim 1wherein said feed hopper is provided with stirring means for admixingthe raw materials received therein.
 3. An extrusion apparatus accordingto claim 1 wherein said screw is a single cylindrical screw of constantpitch.
 4. An extrusion apparatus according to claim 1 wherein said screwis a single tapered screw of changing pitch from the feed hopper to thedie in order to maintain a constant shear and compression on the rawmaterials.
 5. An extrusion apparatus according to claim 2 wherein saidextrusion chamber has a constant screw channel depth.
 6. An extrusionapparatus according to claim 1 wherein said heating means comprise aheating jacket surrounding the discharge end of the extrusion chamberfor a length sufficient to heat the raw materials to a desiredtemperature.
 7. An extrusion apparatus according to claim 1 wherein saidcooling means comprise a plurality of heat dissipation fins surroundingsaid holding chamber.
 8. An extrusion apparatus according to claim 1wherein said cooling means comprise a cooling jacket.
 9. An extrusionapparatus according to claim 1 wherein said die is located between theextrusion chamber and the holding chamber at the discharge end of saidscrew conveyor.
 10. An extrusion apparatus according to claim 1 whereinthe desired compression ratio is within a range of from about 1:1 to1:5.